At present, switching power supplies have been extensively used. For applications with input power below 75 W and having no requirement on power factor (PF), Fly-back switching power supplies may have fascinating advantages, which have been presented in Chinese patent applications No. 201510753470.X and No. 201510753530.8, both entitled “RIPPLE CURRENT GENERATING CIRCUIT”. Moreover, the two applications also introduce that there is no effective testing method and instrument to control the rated ripple current of an electrolytic capacitor at present and present technical solutions to provide a ripple current generating circuit that can provide a high-frequency ripple current to test an electrolytic capacitor at low cost and low energy consumption. For convenience, the “electrolytic capacitor” is hereinafter referred to as a “capacitor”.
The two patent applications cited above are hereinafter referred to as existing patent A (201510753470.X) and existing patent B (201510753530.8) respectively, in an order that they are presented herein. The solutions of the two above patent applications still have problems: when a tested capacitor is about to be out of operation and the Equivalent Series Resistance (ESR) thereof increases greatly, the performance of the capacitor is already close to be out of use. If the capacitor is not shut down at this point, the capacitor will be in an accelerated out-of-operation mode. Once the capacitor is out of operation, the body of the capacitor can be exploded, and the electrolyte in the capacitor will fly to everywhere, which is difficult to clean. Furthermore, many associated failures will be caused, so that losses are added. For example, a field-effect transistor Q may be exploded, which may result in shadow over the hearts of the operators of test equipment and thus stop them from operations on equipment.
In the existing patent A, a direct-current power supply and an inductor are used for supplying power. A transformer, a diode, a field-effect transistor and a pulse width modulation control circuit constitute a core main body of a circuit according to a method set forth in the claims, and the function of the circuit is to generate a ripple current and return the electric energy consumed when generating the ripple current to the direct-current power supply or a tested capacitor through a second winding nearly without loss. For convenience, this part of circuit for generating the ripple current and returning the energy nearly without loss is referred to as a lossless ripple current generator circuit, a lossless ripple current generator for short.
In the existing patent B, a direct-current power supply and a first inductor are used for supplying power. A second inductor, a first diode, a second diode, two field-effect transistors and a pulse width modulation control circuit constitute a core main body of a circuit according to a method set forth in the claims, and the function of the circuit is to generate a ripple current and return the electric energy consumed when generating the ripple current to the direct-current power supply or a tested capacitor through the second inductor, the first diode and the second diode nearly without loss. For convenience, this part of circuit for generating the ripple current and returning the energy nearly without loss is also referred to as a lossless ripple current generator circuit, a lossless ripple current generator for short too.
The technical terms “lossless ripple current generator” present below and in the claims has the same meaning with relevant circuits in the above existing patents A and B, i.e., a circuit for generating a ripple current and returning the electric energy consumed when generating the ripple current to a direct-current power supply or a tested capacitor nearly without loss.
Further, this problem is solved in the patent applications No. 201610040377.9 and 201610040376.4. The technical solutions for this problem are summarized as follows: an indicating circuit is interposed between a direct-current power supply U and a tested capacitor circuit. The indicating circuit is composed of an inductor L and a light-emitting diode (LED) connected in parallel. The current direction of supplying power to the outside by the direct-current power supply U is opposite to the forward switch-on direction of the LED. When the tested capacitor performs normally, the exciting current of the switching transistor Q basically does not appear in the inductor L and the LED does not emit light. When the ESR of the tested capacitor increases greatly, the exciting current of Q appears in L, and when Q is switched off, the exciting current flowing through L cannot change suddenly. A follow current is generated by the light emitted LED, and meanwhile, the LED is driven to emit light, thus prompting a user.
The latter two patent applications cited above are referred to as existing patent C (201610040377.9) and existing patent D (201610040376.4), respectively, in an order that they are presented herein. The existing patent C and the existing patent D also have problems.
An inductor L is used for isolating a power supply from a tested capacitor, so that the direct-current power supply only provides direct current and the tested capacitor provides a high-frequency ripple current. The inductor L is required to have high inductance. However, the inductor L also needs to be used for realizing an indicating circuit and thus is required to have appropriate inductance, because high inductance may cause problem to the indicating circuit. That is, the indicating circuit may start to indicate when the ESR of the tested capacitor increases slightly, which is inconvenient to use.
Therefore, it is desirable to provide a ripple current generating circuit comprising an inductor L that is only used for isolating a power supply so that the direct-current power supply only provides direct current. The inductor L may not provide an indicating signal. The indicating signal for the capacitor failure is provided by other circuit for immediate shutdown by equipment or an operator.